1. Field of the Invention
The present invention relates to brushless DC motors, and in particular to a brushless DC motor which uses a single magnetic sensor for sensing position of the rotor.
2. Description of the Prior Art
Brushless DC motors have been developed and used in the past. One proposed construction is illustrated in U.S. Pat. No. 3,486,099 by Brunner et al, which uses two Hall effect sensors which control energization of four windings in order to generate a rotating magnetic field.
In the past, brushless DC motors have also been proposed which use only a single Hall sensor. This type of brushless DC motor would be much less expensive due to fewer windings, fewer Hall sensors, and simpler control circuitry. This type of brushless DC motor, however, has a significant defect.
In particular, the proposed prior art single sensor brushless DC motor typically has a rotor which is half magnetized with a north polarity and half magnetized with a south polarity. The magnetic periodic angle of the stator (which is mounted inside the rotor) is 180.degree.. The stator has a pair of magnetic poles which are located close to the inside of the rotor and are located 180.degree. from one another. If only one magnetic sensor (for example a Hall sensor) is used to detect the rotor location, the correlation between the torque will be zero at an angle of 180.degree. and 360.degree.. If the motor is stopped with the rotor at either of these two points of zero torque, the motor will not restart, since no torque is applied to the rotor. As a result, a motor with this configuration is of little practical use.
In an attempt to overcome this problem while using a single magnetic sensor, a brushless DC motor is proposed in U.S. Pat. No. 3,873,897 by Muller in which the air gap between the rotor and the stator varies as a function of angular degrees.
There is a continuing need for improved brushless DC motors in which only a single rotor position magnetic sensor is used and in which the point of zero torque is eliminated. It is further desirable that improved brushless DC motors be developed having an organization and construction which makes the cost significantly less than prior art brushless DC motors.